Coverlay for high-frequency circuit substrate

ABSTRACT

To provide a coverlay for a high-frequency circuit substrate, that uses polyimide film and fluororesin, has excellent mechanical properties and heat resistance, and can increase workability during the manufacture of high-frequency circuit substrates. Resolution Means: The coverlay for a high-frequency circuit substrate including a polyimide film and a fluororesin bonded together, and an adhesive strength between the polyimide film layer and the fluororesin layer being greater than 3.0 N/cm.

BACKGROUND OF THE DISCLOSURE

The present invention pertains to a coverlay for a high-frequencycircuit substrate.

Printed wiring boards are used widely in electronic and electricalequipment. Among them, flexible printed wiring boards that can be bentare widely used in the bending parts of personal computers, portabletelephones, and the like, and in parts that require bending, such ashard disks.

As substrates for such flexible printed wiring boards and substrates forcoverlays for protecting printed wiring boards, normally various typesof polyimide film are used in consideration of such properties as heatresistance, dimensional stability, flexibility, high bendability, andease of making into a thin film, and most coverlays are made of acombination of polyimide film and adhesive.

With the growth in the volume of information being transmitted in recentyears, there is an increasing demand for circuit substrates for highfrequency applications. As the frequency used for transmissionincreases, the increase in frequency is also accompanied by moretransmission loss. Because circuits with high transmission loss are notpractical, in order to transmit information efficiently at highfrequencies it is necessary to reduce the transmission loss.

Transmission loss can be reduced by lowering the dielectric constant ofthe substrate or coverlay, and because of this, low-dielectric-constantsubstrates and coverlays are in demand. Normally the dielectric constantof the polyimide film that is used in the substrates and coverlays offlexible printed circuit boards is 3.0 to 3.5, which is insufficient fora low-dielectric-constant material.

So as a way to reduce the dielectric constant, methods have beendeveloped in which a fluororesin of low dielectric constant is laminatedbetween the copper and polyimide layer in the circuit wires that areused in the substrate (the copper foil is etched on the side of acopper-clad laminate (CCL)) (see Japanese Patent No. 2890747 andJapanese Patent No. 4917745 (Patent Documents 1 and 2)).

But in the process of manufacturing a circuit substrate using thecoverlay, if for example, kiss lamination is done by aligning thepositions of a copper-clad laminate and a coverlay, in some cases, sometime may be needed for the aligning in order to prevent attachmentmisalignment, and further improvements in manufacturing efficiency havebeen requested for industrial implementation.

From such considerations, it has been desired that a coverlay bedeveloped that uses fluororesin, and has as a base material a newpolyimide film that is easy to process and is a low dielectric constantmaterial for use in high-frequency circuit substrates.

-   Patent Document 1: Japanese Patent No. 2890747-   Patent Document 2: Japanese Patent No. 4917745

SUMMARY

An object of the present invention is to provide a coverlay for ahigh-frequency circuit substrate that employs polyimide film andfluororesin, has excellent mechanical properties and heat resistance,and can improve workability during manufacture of a high-frequencycircuit substrate.

That is, the present invention pertains to the following invention.

1. A coverlay for a high-frequency circuit substrate, the coverlaycomprising a polyimide film and a fluororesin bonded together, and anadhesive strength between the polyimide film layer and the fluororesinlayer being greater than 3.0 N.

2. The coverlay according to 1 above, wherein a thermal shrinkagethereof at 260° C. for 30 minutes is less than ±0.1%.

3. The coverlay according to 1 or 2 above, wherein the fluororesin has amelting point of 200° C. or less.

4. The coverlay according to any of 1 to 3 above, wherein thefluororesin is a fluorine-containing ethylenic polymer, and thefluorine-containing ethylenic polymer contains a carbonyl group.

5. The coverlay according to 4 above, wherein a quantity of carbonylgroups contained in the fluorine-containing ethylenic polymer totals 3to 1000 groups per 1×10⁶ main-chain carbon atoms.

6. The coverlay according to any of 1 to 3 above, wherein thefluororesin is made up of fluorine-containing ethylenic polymer that hasat least one type selected from a group made up of carbonate groups,carboxylic acid halide groups, and carboxylic acid groups totaling 3 to1000 groups per 1×10⁶ main-chain carbon atoms.

7. The coverlay according to any of 1 to 3 above, wherein thefluororesin is one or more types of fluorine-containing ethylenicmonomer selected from a group made up of tetrafluoroethylene, vinylidenefluoride, chlorotrifluoroethylene, vinyl fluoride, hexafluoropropylene,hexafluoroisobutene, monomers represented by the following formula (X):

CH₂═CR¹(CF₂)_(n)R²  (X)

(wherein R¹ represents H or F, R² represents H, F, or Cl, and n is apositive integer in the range of 1 to 10), and perfluoro(alkyl vinylethers) having 2 to 10 carbon atoms, or a fluorine-containing ethylenicpolymer made by polymerizing the fluorine-containing ethylenic monomerand an ethylenic monomer having 5 or fewer carbon atoms.

8. The coverlay according to any one of 1 to 3 above, wherein thefluororesin is a copolymer made by polymerizing at least the following(a), (b), and (c).

(a) 20 to 90 mol % of tetrafluoroethylene,

(b) 10 to 80 mol % of ethylene, and

(c) 1 to 70 mol % of a compound represented by the formula:

CF₂═CFR³  (Y)

(wherein R³ represents CF₃ or OR⁴, and R⁴ represents a perfluoroalkylgroup having 1 to 5 carbon atoms).

9. The coverlay according to any of 1 to 8 above, wherein the polyimidefilm is made up mainly of one or more aromatic diamine componentsselected from a group made up of paraphenylene diamine,3,4′-diaminodiphenyl ether, and 4,4′-diaminodiphenyl ether, and one ormore acid anhydride components selected from a group made up ofpyromellitic acid dianhydride and 3,3′,4,4′-biphenyl tetracarboxylicacid dianhydride.

The high-frequency circuit substrate coverlay of the present inventionis industrially useful because, with its excellent mechanical propertiesand heat resistance, it enhances workability during the manufacture ofhigh-frequency circuit substrates. Also, with a high-frequency circuitsubstrate that employs the coverlay of the present invention, thelow-dielectric-constant fluororesin can be made so that the dielectricconstant is low and the transmission loss is kept in check.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows the results of measuring the transmission properties ofcircuits employing the coverlays of the working examples and of thecomparison examples.

DETAILED DESCRIPTION

The coverlay of the present invention for a high-frequency circuitsubstrate is a coverlay that is made up of a polyimide film and afluorosein bonded together, and the adhesive strength between thepolyimide film layer and the fluororesin layer (initial adhesive force)exceeds 3.0 N/cm.

In manufacturing the polyimide film that is used in this coverlay,first, a polyamic acid solution is prepared by polymerizing an aromaticdiamine component and an acid anhydride component in an organic solvent.

Specific examples of the aromatic diamine component includeparaphenylene diamine, metaphenylene diamine, benzidine, paraxylilenediamine, 4,4′-diaminodiphenyl ether, 3,4′-diaminodiphenyl ether,4,4′-diaminodiphenyl methane, 4,4′-diaminodiphenyl sulfone,3,3′-dimethyl-4,4′-diaminodiphenyl methane, 1,5-diaminonaphthalene,3,3′-dimethoxybentidine, 1,4-bis (3 methyl-5 aminophenyl)benzene, andamidic derivatives thereof. Among these, it is preferable for circuitsubstrate applications to adjust the amounts of the diamines such asparaphenylene diamine, 3,4′-diaminodiphenyl ether, and the like, whichare effective in increasing the tensile strength of the film, so thatthe tensile strength of the polyimide film that is ultimately obtainedis 3.0 GPa or more. Of these aromatic diamines, paraphenylene diamine,4,4′-diaminodiphenyl ether, and 3,4′-diaminodiphenyl ether arepreferable. These may be used either singly as one type, or with two ormore type thereof mixed together. If paraphenylene diamine and4,4′-diaminodiphenyl ether and/or 3,4′-diaminodiphenyl ether are usedtogether, there are no particular restrictions on their blending ratio(mole ratio), but it is preferable that the ratio of4,4′-diaminodiphenyl ether and/or 3,4′-diaminodiphenylether:paraphenylene diamine=69:31 to 100:0 (except 0), and morepreferable that it be 70:30 to 90:10.

Specific examples of the acid anhydride component include pyromelliticacid, 3,3′,4,4′-biphenyl tetracarboxylic acid, 2,3′,3,4′-biphenyltetracarboxylic acid, 3,3′,4,4′-benzophenone tetracarboxylic acid,2,3,6,7-naphthalene dicarboxylic acid,2,2-bis(3,4-dicarboxylphenyl)ether, pyridine-2,3,5,6-tetracarboxylicacid, and acid anhydrides of amidic derivatives thereof and the like.Among these acid anhydrides, pyromellitic acid, 3,3′,4,4′-biphenyltetracarboxylic acid, 2,3′,3,4′-biphenyl tetracarboxylic acid arepreferable. These may be used either singly as one type, or with two ormore types mixed together. As the mole ratio of the acid anhydridecomponents, if pyromellitic acid dianhydride and 3,3′,4,4′-biphenyltetracarboxylic acid dianhydride are used together, there are noparticular restrictions on their blending ratio (mole ratio), but it ispreferable that the ratio of pyromellitic aciddianhydride:3,3′,4,4′-biphenyl tetracarboxylic acid dianhydride=0(except 0):100 to 97:3, and more preferable that it be 30:70 to 95:5.

Examples of the polyimide film to be used in the coverlay of the presentinvention preferably include mainly ones that are made up of one or morearomatic diamine components selected from a group made up ofparaphenylene diamine, 3,4′-diaminodiphenyl ether, and4,4′-diaminodiphenyl ether, and one or more acid anhydride componentsselected from a group made up of pyromellitic acid dianhydride and3,3′,4,4′-biphenyl tetracarboxylic acid dianhydride.

As organic solvents that can be used for forming the polyamic acidsolution in the present invention, examples include dimethyl sulfoxide,diethyl sulfoxide, and other sulfoxide solvents; N,N-dimethylform[amide], N,N-diethyl formamide, and other formamide solvents;N,N-dimethyl acetoamide, N,N-diethyl acetoamide, and other acetoamidesolvents; N-methyl-2-pyrrolidone, N-vinyl-2-pyrrolidone, and otherpyrrolidone solvents; phenol, o-, m-, or p-cresol, xylenol, phenolhalide, catechol, and other phenol solvents; or hexamethylphosphoramide, γ-butyrolactone, and other aprotic polar solvents; it isdesirable to use these either singly or as a mixture of two or moretypes, and in addition, aromatic hydrocarbons such as xylene or toluenemay be used.

There are no particular restrictions on the polymerization method; anywell known method may be used, such as (1) a method of polymerizing inwhich first the entire amount of the aromatic diamine component isinserted into the solvent, and the acid anhydride component is thenadded so that it becomes an amount equivalent to the entire amount ofthe aromatic diamine component.

(2) A method of polymerizing in which first the entire amount of theacid anhydride component is inserted into the solvent, then the aromaticdiamine component is added so that it becomes an amount equivalent tothe acid anhydride component.

(3) A method of polymerizing in which a first aromatic diamine component(a1) is inserted into the solvent, then a first acid anhydride component(b1) is mixed at a ratio that becomes 95 to 105 mol % with respect tothe reaction components for the time needed for a reaction to occur,after which a second aromatic diamine component (a2) is added, followingwhich a second acid anhydride component (b2) is added so that the entirearomatic diamine component and the entire acid anhydride componentbecome roughly equivalent amounts.

(4) A method of polymerizing in which a first acid anhydride component(b1) is inserted into the solvent, then a first aromatic diaminecomponent (a1) is mixed at a ratio that becomes 95 to 105 mol % withrespect to the reaction components for the time needed for a reaction tooccur, after which a second acid anhydride component (b2) is added,following which a second aromatic diamine component (a2) is added sothat the entire aromatic diamine component and the entire acid anhydridecomponent become roughly equivalent amounts.

(5) A method wherein, in a solvent, a polyamic acid solution (A) isprepared by causing reactions such that one or the other of the firstaromatic diamide component and an acid anhydride component is in excess,and in a separate solvent, a polyamic acid solution (B) is prepared bycausing reactions so that one or the other of the second aromaticdiamine component and an acid anhydride component is in excess, and thenthe polyamic acid solutions (A) and (B) thereby obtained are mixedtogether and the polymerization is completed. If when preparing thepolyamic acid solution (A), the aromatic diamine component is in excess,then with the polyamic acid solution (B) the acid anhydride component ismade in excess, or when with the polyamic acid solution (A), the acidanhydride component is in excess, then the aromatic diamine component ismade in excess with the polyamic acid solution (B). The polyamic acidsolutions (A) and (B) are mixed together, and an adjustment is made sothat the total aromatic diamine component and acid anhydride componentto be used in these reactions are of roughly equivalent amounts.

Also, the polymerization methods are not limited thereto, and one mayalso use any other well known method.

The polyamic acid solution that is thus obtained normally contains asolid portion of 5 to 40 wt %, and preferably 10 to 30 wt %. Moreover,its viscosity is normally 10 to 10,000 Pa·s as measured by a Brookfieldviscometer, and is preferably 300 to 5,000 Pa·s for sake of stableliquid feeding. It is acceptable for the polyamic acid in an organicsolvent solution to be partially imidized.

Next, the method for manufacturing the polyimide film of the presentinvention, which employs the above polyamic acid solution, is described.

Examples as methods for making the polyimide film include a method inwhich the polyimide film is obtained by casting the polyamic acidsolution in film form and then thermally removing rings and the solvent,and a method in which the polyimide film is obtained by mixing a ringremoval catalyst and dehydrating agent into the polyamic acid solutionto chemically remove rings and make a gel film, which is then heated toremove the solvent.

The polyamic acid solution may contain a ring removal catalyst(imidization catalyst), dehydrating agent, gelling delaying agent, andthe like.

Specific examples of the ring removal catalysts to be used in thepresent invention include trimethyl amine, triethylene diamine, andother aliphatic tertiary amines; dimethyl aniline and other aliphatictertiary amines; and isoquinoline, pyridine, beta picoline, and otherheterocyclic tertiary amines, and the like, but heterocyclic tertiaryamines are preferable. These may be used either singly as one type or asa mixture of two or more types.

Specific examples of the dehydrating agents to be used in the presentinvention include acetic anhydride, propionic anhydride, butyricanhydride, and other aliphatic carboxylic anhydrides, and benzoicanhydride and other aromatic carboxylic anhydrides, and the like, butacetic anhydride and/or benzoic anhydride are preferable.

As a method for manufacturing polyimide film from a polyamic acidsolvent, examples include a method in which a polyamic acid solutionthat is made to contain the ring removal catalyst and the dehydratingagent is made to flow from a slitted nozzle onto a support body and ismolded into a film form, imidization on the support body is allowed toproceed partway, a gel film that can support itself is made, then it ispeeled off from the support body, is heated, dried, and imidized, and isheat-treated.

The “support body” is a metal rotating drum or endless belt, and itstemperature is controlled by a liquid or gas heat transfer medium and/oran electric heater or other radiant heat.

The gel film is heated normally to 30 to 200° C., and preferably to 40to 150° C. by being heated by the support body and/or by being heated byhot air, an electric heater, or another heat source, thereby causingring closure reactions, and by drying the volatile component, such asthe organic solvent, that is set free, the film acquires the ability tosupport itself, and is peeled away from the support body.

The gel film that is peeled from the support body may as necessaryundergo a stretching extension treatment in the running direction whileregulating the running speed by a rotating roll. The extensionmagnification (MDX) in the mechanical conveyance direction and theextension magnification (TDX) in the direction perpendicular to themechanical conveyance direction are implemented at 1.01 to 1.9-times,and preferably at 1.05 to 1.6-times.

The film dried in the drying zone is heated from 15 seconds to 10minutes by hot air, an infrared heater, or the like. Next, it isheat-treated for 15 seconds to 20 minutes at a temperature of 250 to500° C. by hot air and/or an electric heater, or the like.

Also, the running speed is adjusted to adjust the thickness of thepolyimide film, and the thickness of the polyimide film is normallyabout 2 to 250 μm, and preferably about 2 to 100 μm. Thicknesses thinneror thicker are undesirable because that would significantly degrade themanufacturability of the film.

As the polyimide film to be used in the present invention, commerciallyavailable products may be used. There are no particular restrictions onthe commercially available products, and examples include Capton ENtypes (for example, 50EN-S (brand name, made by Dupont-Toray Co., Ltd.),100EN (brand name, made by Dupont-Toray Co., Ltd.), and the like),Capton H types (for example, Capton 100H (brand name, made byDupont-Toray Co., Ltd.), etc.), and the like.

The polyimide film in the present invention may include a plasticizer orother resin, or the like to the extent that doing so does not detractfrom the purpose of the present invention.

For the plasticizers, there are no particular restrictions, and examplesinclude hexylene glycol, glycerin, β-naphthol, dibenzyl phenol, octylcresol, bisphenol A, and other bisphenol compounds; p-hydroxyoctylbenzoate, p-hydroxy benzoic acid-2-ethyl hexyl, p-hydroxy benzoic acidpeptyl, p-hydroxy benzoic acid ethylene oxide and/or propylene oxideadducts, ε-caprolactone, phosphoric acid ester compounds of phenols,N-methyl benzene sulfonamide, N-ethyl benzene sulfonamide, N-butylbenzene sulfoneamide, toluene sulfonamide, N-ethyl toluene sulfonamide,N-cyclohexyl toluene sulfonamide, and the like.

As the other resins to be blended into the polyimide, those havingsuperior compatibility are preferable, and examples include ester and/orcarboxylic acid modified olefin resin, acrylic resin (in particular,acrylic resin that has a glutarimide group), ionomer resin, polyesterresin, phenoxy resin, ethylene-propolyene-diene copolymer, polyphenyleneoxide, and the like.

The polyimide film in the present invention may include colorants andvarious types of additives, insofar as this would not detract from thepurpose of the present invention. As the additives, examples includeantistatic agents, flame retardants, heat stabilizers, ultraviolet rayabsorbents, lubricants, mold release agents, crystal nucleus agents,reinforcing agents (fillers), and the like. Also, the surface of thepolyimide film may be coated with ink, and the like.

There are no particular restrictions on the fluororesin that is used inthe coverlay of the present invention, but a fluorine-containingethylenic polymer is preferable. In the fluorine-containing ethylenicpolymer in the present invention, a carbonyl group or a functional groupthat contains a carbonyl group is joined to the fluorine-containingethylenic polymer chain.

The “carbonyl group” means a functional group having —C(═O)— that canbasically react with the imide groups or amino groups in the polyimidefilm. Specifically examples include carbonates, carboxylic acid halides,aldehydes, ketones, carboxylic acid, esters, acid anhydrides, isocyanategroups, and the like. There are no particular restrictions on thecarbonyl group, but preferable for ease of introduction and highreactivity with the polyamide resin are carbonate groups, carboxylicacid halide groups, carboxylic acid groups, ester groups, and acidanhydride groups, and more preferable are carbonate groups andcarboxylic acid halide groups.

The number of carbonyl groups in the fluorine-containing ethylenicpolymer in the present invention can be suitably selected according todifferences in the types of materials that are laminated together, theshape, the purpose of the adhesion, the application, the adhesive forcethat is required, the mode of polymerization, and the method ofadhesion, and the like, but it is preferable that the number of carbonylgroups total 3 to 1000 groups per 1×10⁶ carbon atoms in the main chain.If the number of the carbonyl groups per 1×10⁶ carbon atoms in the mainchain is less than 3, sometimes there will not be sufficient adhesiveforce. Moreover, if it is greater than 1000, sometimes the adhesiveforce will be reduced due to chemical changes of the carbonyl groupsduring the adhesion operation. More preferable is 3 to 500, even morepreferable is 3 to 300, and particularly preferable is 5 to 150. Also,the amount of carbonyl groups in the fluorine-containing ethylenicpolymer can be measured by infrared absorption spectrum analysis.

Therefore if the fluorine-containing ethylenic polymer of the presentinvention for example is one that has carbonate groups and/or carboxylicacid halide groups, if it has carbonate groups, it is preferable thatthe number of carbonate groups be 3 to 1000 per 1×10⁶ carbon atoms inthe main chain, and if the fluorine-containing ethylenic polymer of thepresent invention has carboxylic acid halide groups, it is preferablethat the number of carboxylic acid halide groups be 3 to 1000 per 1×10⁶carbon atoms in the main chain. If the fluorine-containing ethylenicpolymer of the present invention has both carbonate groups andcarboxylic acid halide groups, it is preferable that the total number ofcarbonate groups and carboxylic acid halide groups be 3 to 1000 per1×10⁶ carbon atoms in the main chain. If the number of the carbonategroups and/or carboxylic acid halide groups is less than 3 per 1×10⁶carbon atoms in the main chain, sometimes there will not be sufficientadhesive force. Moreover, if it is greater than 1000, sometimes, due tochemical changes of the carbonate groups or carboxylic acid halidegroups during the adhesion operation, the production of gas coming fromthe adhesive interface will have an adverse effect, and the adhesiveforce will be reduced. From the standpoint of heat resistance andresistance to chemicals, it is more preferable that it be 3 to 500, evenmore preferable that it be 3 to 300, and particularly preferable that itbe 5 to 150. Also, if carboxylic acid halide groups, which haveexcellent reactivity with polyamide resin, are present in thefluorine-containing ethylenic polymer in a quantity of 10 or more, andmore preferably 20 or more per 1×10⁶ carbon atoms in the main chain,then even if the quantity contained in the total of carbonyl groups isless than 150 per 1×10⁶ carbon atoms in the main chain, excellentadhesion can be obtained with a layer (A) made up of polyamide resin.

The carbonate group in the fluorine-containing ethylenic polymer in thepresent invention is generally a group that has a —OC(═O)O— bond;specifically, it is one with a structure of an —OC(═O)O—R group (where Ris an organic group (for example, a C₁ to C₂₀ alkyl group (andpreferably a C₁ to C₁₀ alkyl group), a C₂ to C₂₀ alkyl group having anether bond, or the like) or a group VII element). As carbonate groups,preferable examples include —OC(═O)OCH₃, —OC(═O)OC₃H₇, —OC(═O)OC₈H₁₇,—OC(═O)OCH₂CH₂CH₂OCH₂CH₃, and the like.

The carboxylic acid halide group in the fluorine-containing ethylenicpolymer in the present invention is specifically one of a —COY structure(where Y is a halogen element), and examples include —COF and —COCl.

A fluorine-containing ethylenic polymer that has these carbonyl groupscan itself retain the excellent properties of a fluorine-containingresin, and can confer them to the laminate after formation, with nodiminution in such excellent properties that a fluorine-containing resinhas.

The fluorine-containing ethylenic polymer in the present inventionincludes carbonyl groups in its polymer chain, but there are noparticular restrictions on how the carbonyl groups are contained in thepolymer chain; for example a carbonyl group or a functional group thatcontains a carbonyl group may be joined to the end of the polymer chainor to a side chain. Among them, those that have a carbonyl group on theend of the polymer chain are preferable, because they do notsignificantly reduce heat resistance, mechanical properties, orresistance to chemicals, or because they are beneficial from aproductivity and cost perspective. Here, a mode that is preferable,because it is very easy to introduce and because it is also easy tocontrol the quantity introduced, is the method in which either carbonylgroups are included such as peroxy carbonate or peroxy ester, or thecarbonyl groups are introduced to the ends of the polymer chain using apolymerization initiator that has a functional group that can be changedinto a carbonyl group. Also, in the present invention, a carbonyl groupthat originates in a peroxide means a carbonyl group that is introduceddirectly or indirectly from a functional group that is included in theperoxide.

Also, in the fluorine-containing ethylenic polymer in the presentinvention, even if a fluorine-containing ethylenic polymer that does notcontain carbonyl groups is present, it suffices that as an overallpolymer, it have a number of carbonyl groups in the above range as atotal per 1×10⁶ carbon atoms in the main chain.

In the present invention, the type and structure of thefluorine-containing ethylenic polymer can be suitably selected accordingto the purpose, the application, and the method of use, but here it ispreferable that its melting point be 160 to 270° C. With such a polymer,this is advantageous especially if the lamination is done by a heating,melting, sticking-together process, because, in particular, sufficientadhesiveness can be obtained between the carbonyl groups and the othermaterial, and a strong adhesive force with the other material can beobtained. From the perspective of enabling lamination to an organicmaterial of relatively low heat resistance, the melting point is morepreferably 250° C. or less, even more preferably 230° C. or less, andparticularly preferably 200° C. or less. For the melting point, using aSeiko model DSC device (made by Seiko Electronics Co.), the melting peakwas recorded when the temperature was increased at a rate of 10° C./min,and the temperature corresponding to the maximum value was taken as themelting point (Tm).

With regard to the molecular weight of the fluorine-containing ethylenicpolymer in the present invention, it is preferable that it be in a rangein which the polymer can be formed below the thermal breakdowntemperature and that the resulting formed body be able to exhibit theexcellent mechanical properties that are characteristic of thefluorine-containing ethylenic polymer. Specifically, taking the meltflow rate (MFR) as an index of the molecular weight, it is preferablethat the MFR be 0.5 to 100 g/10 min at any temperature in the range ofabout 230 to 350° C., which is the general molding temperature range forfluororesin. For the MFR, using a melt indexer (made by Toyo SeikiSeisaku-Sho, Ltd.), measurements were taken at unit intervals (10-minuteintervals) of the weight (g) of the polymer flowing out of a 2 mmdiameter nozzle having a length of 8 mm under a load of 5 kg at varioustemperatures.

The structure of the fluorine-containing ethylenic polymer chain is ingeneral a homopolymer chain or copolymer chain that has repeated unitsthat are derived from at least one type of fluorine-containing ethylenicmonomer, and it may be a polymer chain made up of either afluorine-containing ethylenic monomer only, or made by polymerizing afluorine-containing ethylenic monomer with an ethylenic monomer thatdoes not have any fluorine atoms.

The fluorine-containing ethylenic monomer is an olefinic unsaturatedmonomer that has fluorine atoms, and specific examples includetetrafluoroethylene, vinylidene fluoride, chlorotrifluoroethylene, vinylfluoride, hexafluoropropylene, hexafluoroisobutene, monomers representedby the formula (X):

CH₂═CR¹(CF₂)_(n)R²  (X)

(wherein R¹ represents H or F, R² represents H, F, or Cl, and n is apositive integer in the range 1 to 10), and perfluoro(alkyl vinylethers) having 2 to 10 carbon atoms, and the like.

The above ethylenic monomer that does not have any fluorine atoms ispreferably selected from ethylenic monomers having 5 or fewer carbonatoms, in order not to reduce the heat resistance, and the like.Specific examples include ethylene, propylene, 1-butene, 2-butene, vinylchloride, vinylidene chloride, and the like.

If a fluorine-containing ethylenic monomer and an ethylenic monomer thatdoes not have any fluorine atoms are used, the composition of themonomers may have a weight ratio such that the fluorine-containingethylenic monomer is 10 mol % or more to less than 100 mol % (forexample, 30 mol % or more to less than 100 mol %), and the ethylenicmonomer that does not have any fluorine atoms is greater than 0 mol % tono greater than 90 mol % (for example, greater than 0 mol % to nogreater than 70 mol %).

In the fluorine-containing ethylenic polymer in the present invention,the melting point or glass transition point of the polymer can beadjusted by selecting the type, combination, composition ratio, andother properties of the fluorine-containing ethylenic monomer and theethylenic monomer that does not have any fluorine atoms.

As the fluorine-containing ethylenic polymer in the present invention,preferable for heat resistance and resistance to chemicals is afluorine-containing ethylenic polymer that contains a carbonyl group inwhich a tetrafluoroethylene unit is an essential component, andpreferable for formability and workability is a fluorine-containingethylenic copolymer that contains a carbonyl group in which a vinylidenefluoride unit is an essential component.

Preferable specific examples of the fluorine-containing ethylenicpolymer in the present invention include fluorine-containing ethyleniccopolymers (I) to (V) that contain a carbonyl group in which thefluorine-containing ethylenic polymer is essentially made bypolymerizing the following monomers:

(I) a copolymer made by polymerizing at least tetrafluoroethylene andethylene,(II) a copolymer made by polymerizing at least tetrafluoroethylene and acompound represented by the formula (Y):

CF₂═CFR³  (Y)

(wherein R³ represents CF₃ or OR⁴, and R⁴ represents a perfluoroalkylgroup having 1 to 5 carbon atoms),(III) a copolymer made by polymerizing at least vinylidene fluoride,(IV) a copolymer made by polymerizing at least (a), (b), and (c) below,(a) tetrafluoroethylene 20 to 90 mol %(b) ethylene 10 to 80 mol %(c) 1 to 70% of a compound expressed by

CF₂═CFR³  (Y)

(where R³ has the same meaning as above), and(v) a copolymer made by polymerizing at least (d), (e), and (f) below.(d) vinylidene fluoride 15 to 60 mol %(e) tetrafluoroethylene 35 to 80 mol %(f) hexafluoropropylene 5 to 30 mol %Other known monomers may be added to these specific examples, includingthe above monomers, insofar as they do not interfere with the effects ofthe present invention.

Each of these examples of a fluorine-containing ethylenic polymer thatcontains a carbonyl group is preferable especially in that it hasexcellent heat resistance.

Examples of the copolymer (I) include polymer-chain carbonylgroup-containing copolymers that, with respect to the monomers as awhole excluding monomers that have a carbonyl group (if it has afunctional group that has a carbonyl group on a side chain), are made upof 20 to 90 mol % tetrafluoroethylene units (for example, 20 to 60 mol%), 10 to 80 mol % ethylene units (for example, 20 to 60 mol %), and 0to 70 mol % of other monomers that can be copolymerized therewith.

As other monomers that can be copolymerized, examples includehexafluoropropylene, chlorotrifluoroethylene, and monomers representedby the formula (X):

CH₂═CR¹(CF₂)_(n)R²  (X)

(wherein R¹ represents H or F, R² represents H, F, or Cl, and n is apositive integer in the range 1 to 10), and perfluoro(alkyl vinylethers) having 2 to 10 carbon atoms, and normally these are used as asingle types or as two or more types together.

As the copolymer (I), the following can be suitably listed in that theymaintain excellent performance in a tetrafluoroethylene/ethylenecopolymer, they can be made to have a relatively low melting point aswell, and they can exhibit maximum adhesion with other materials.

(I-1) Polymer-chain carbonyl group-containing copolymers made up of 62to 82 mol % tetrafluoroethylene units, 20 to 38 mol % ethylene units,and 0 to 10 mol % of other monomer units,(I-2) Polymer-chain carbonyl group-containing copolymers made up of 20to 80 mol % tetrafluoroethylene units, 10 to 80 mol % ethylene units, 0to 30 mol % hexafluoropropylene units, and 0 to 10 mol % of othermonomer units.

Suitable examples of the copolymer (II) include the following.

(II-1) Polymer-chain carbonyl group-containing copolymers made up of 65to 95 mol % (preferably, 75 to 95 mol %) tetrafluoroethylene units and 5to 35 mol % (preferably, 5 to 25 mol %) hexafluoropropylene units,(II-2) polymer-chain carbonyl group-containing copolymers made up of 70to 97 mol % tetrafluoroethylene units and 3 to 30 mol % CF₂═CFOR⁴ (whereR⁴ is a perfluoroalkyl group having 1 to 5 carbon atoms) units,(II-3) and a polymer-chain copolymer having carbonyl groups that is madeup of tetrafluoroethylene units, hexafluoropropylene units, andCF₂═CFOR⁴ (where R⁴ is as above) units, wherein the total of thehexafluoropropylene units and the CF₂═CFOR⁴ units is 5 to 30 mol %.

The (II-1) to (II-3) above are also perfluoro copolymers, and even amongfluorine-containing polymers, they are the most excellent in heatresistance, electrical insulation performance, and other properties.

As the copolymer (III), examples include polymer-chain carbonylgroup-containing copolymers that, with respect to the monomer totalexcluding the monomers that have a carbonyl group (if they have acarbonyl group-containing functional group in a side chain), are made upof 15 to 99 mol % vinylidene fluoride units, 0 to 80 mol %tetrafluoroethylene units, and 0 to 30 mol % of either one or more typesof hexafluoropropylene or chlorotrifluoroethylene units. As specificexamples of the copolymer (III), the following can be suitably listed.

(III-1) Polymer-chain carbonyl group-containing copolymers made up of 30to 99 mol % vinylidene fluoride units and 1 to 70 mol %tetrafluoroethylene units,(III-2) polymer-chain carbonyl group-containing copolymers made up of 60to 90 mol % vinylidene fluoride units, 0 to 30 mol % tetrafluoroethyleneunits, and 1 to 20 mol % chlorotrifluoroethylene units,(III-3) polymer-chain carbonyl group-containing copolymers made up of 60to 99 mol % vinylidene fluoride units, 0 to 30 mol % tetrafluoroethyleneunits, and 5 to 30 mol % hexafluoropropylene units,(III-4) polymer-chain carbonyl group-containing copolymers made up of 15to 60 mol % vinylidene fluoride units, 35 to 80 mol %tetrafluoroethylene units, and 5 to 30 mol % hexafluoropropylene units.

There are no particular restrictions on how to manufacture thefluorine-containing ethylenic polymer in the present invention. Thefluorine-containing ethylenic polymer of the present invention can bemanufactured by taking an ethylenic monomer that has a carbonyl groupand copolymerizing it with a fluorine-containing and/or ethylenicmonomer of a type and blend that fits the desired fluorine-containingpolymer. As the ethylenic monomer that has a carbonyl group, preferableexamples include fluorine-containing monomers, such as perfluoroacrylicacid (fluoride), 1-fluoroacrylic acid (fluoride), acrylic acid fluoride,1-trifluoromethacrylic acid (fluoride), perfluorobutenic acid, and thelike; and monomers that do not include fluorine, such as acrylic acid,methacrylic acid, acrylic acid chloride, vinylene carbonate, itaconicacid, citraconic acid, and the like.

On the other hand, various methods can be adopted for obtainingfluorine-containing ethylenic polymer that has a carbonyl group at theend of the polymer molecule, but the method of using peroxide, inparticular peroxycarbonate or peroxy ester, as a polymerizationinitiator can be preferably adopted for its economy and for qualityconsiderations such as heat resistance and resistance to chemicals. Withthis method, a carbonyl group originating in a peroxide (for example, acarbonate group that originates in a peroxide carbonate; an ester groupthat originates in a peroxy ester; or a carboxylic acid halide group orcarboxylic acid group that is obtained by modifying these functionalgroups) can be introduced at the end of a polymer chain. Among thesepolymerization initiators, it is more preferable if peroxide carbonateis used, because the polymerization temperature can be made low andbecause the initiation reactions are not accompanied by side reactions.

As the peroxycarbonates, we can suitably list, for example, compoundsthat are represented by the following formulas (1) to (4):

(wherein R and R^(a) represent a straight-chain or branched monovalentsaturated hydrocarbon group having 1 to 15 carbon atoms, or astraight-chain or branched monovalent hydrocarbon group having 1 to 15carbon atoms that contains an alkoxy group on the end, and R^(b)represents a straight-chain or branched divalent saturated hydrocarbongroup having 1 to 15 carbon atoms, or a straight-chain or brancheddivalent hydrocarbon group having 1 to 15 atoms that contains an alkoxygroup on the end). Particularly preferable are diisopropylperoxycarbonate, di-n-propyl peroxydicarbonate, t-butyl peroxyisopropylcarbonate, bis(4-t-butyl cyclohexyl)peroxydicarbonate, di-2-ethylhexylperoxydicarbonate, and the like.

The amount of peroxycarbonate, peroxy ester, or other initiators that isused varies depending on the type (composition, and the like), molecularweight, and polymerization conditions of the desired polymer and thetype of initiator that will be used, but normally it is preferable thatit be 0.05 to 20 parts by weight, and in particular 0.1 to 10 parts byweight, per 100 parts by weight of the polymer that is obtained by thepolymerization.

As the polymerization method, for industrial purposes, suspensionpolymerization using a fluorine solvent, in an aqueous medium usingperoxycarbonate, or the like as the polymerization initiator, ispreferable, but other polymerization methods may also be adopted, suchas, for example, solution polymerization, emulsion polymerization, bulkpolymerization, and the like. In suspension polymerization, a fluorinesolvent may be used in addition to water. As the fluorine solvent usedin suspension polymerization, one can use, for example,hydrofluorochloroalkanes (for example, CH₃CClF₂, CH₃CCl₂F, CF₃CF₂CCl₂H,CF₂CICF₂CFHCl), chlorofluoroalkanes (for example, CF₂CICFClCF₂CF₃,CF₃CFClCFClCF₃), and perfluoroalkanes (for example,perfluorocyclobutane, CF₃CF₂CF₂CF₃, CF₃CF₂CF₂CF₂CF₃,CF₃CF₂CF₂CF₂CF₂CF₃), and perfluoroalkanes are preferable. There are noparticular restrictions on the amount of fluorine solvent that is used,but in the case of suspension polymerization, for suspendability andeconomy it is preferable to use 10 to 100 wt % with respect to theaqueous medium.

There are no particular restrictions on the polymerization temperature;0 to 100° C. is acceptable. The polymerization pressure is appropriatelyset according to the type, amount, and vapor pressure of the solventthat is used, and the polymerization temperature and otherpolymerization conditions, but 0 to 9.8 MPaG is acceptable.

Also, to adjust the molecular weight, any well known chain transferagent may be used. As chain transfer agents, one may use, for example, ahydrocarbon such as isopentane, n-pentane, n-hexane, cyclohexane, andthe like; an alcohol such as methanol, ethanol, and the like; or ahydrocarbon halide such as carbon tetrachloride, chloroform, methylenechloride, methyl chloride, and the like. Also, the quantity of endcarbonate groups or ester groups it contains can be controlled byadjusting the polymerization conditions, and can be controlled by theamount of peroxycarbonate or peroxy ester that is used, the amount ofchain transfer that is used, the polymerization temperature, and thelike.

Various methods can be adopted to obtain a fluorine-containing ethylenicpolymer that has a carboxylic acid halide group or a carboxylic acidgroup at the end of the polymer molecule; for example, one can obtain itby heating, and causing thermal breakdown (decarboxylation) of afluorine-containing ethylenic polymer that has at its end theaforementioned carbonate group or ester group. The heating temperaturevaries depending on the type of carbonate group or ester group and onthe type of fluorine-containing ethylenic polymer, but normally it is270° C. or more, preferably 280° C. or more, and particularly preferably300° C. or more. Also, it is preferable that the heating temperature bebelow the thermal breakdown temperature of the parts of thefluorine-containing ethylenic polymer other than the carbonate groups orester groups; specifically, 400° C. or less is preferable, and 350° C.or less is more preferable.

A white powder of the resulting fluorine-containing ethylenic polymer orcut pieces of its molten extruded pellets were compression-formed atroom temperature and made into a uniform film of thickness 0.05 to 0.2mm. By infrared spectrum absorption analysis of this film, the peakoriginating in the carbonyl group of the carbonate group (—OC(═O)O—)appeared at an absorption wavelength of 1809 cm⁻¹ (v_(c-o)), and itsabsorbance at the v_(c-o) peak was measured. The number (N) of carbonategroups per 1×10⁶ main-chain carbon atoms was computed by the followingformula (5).

N=500 AW/∈df  (5)

A: absorbance at the v_(c-o) peak of the carbonate group (—OC(═O)O—)∈s: mole absorbance coefficient (l·cm⁻¹·mol⁻¹) of the v_(c-o) peak ofthe carbonate group (—OC(═O)O—). From model compounds, epsilon was setto ∈=170.W: average molecular weight of the monomer as computed from the monomercompositiond: density of the film (g/cm³)f: thickness of the film (mm)Also, in the infrared absorption spectrum analysis, scanning was done 40times using a Perkin-Elmer FTIR spectrometer 1760×(made by Perkin-ElmerCo.). The resulting IR spectrum was used to automatically determine thebaseline using Perkin-Elmer Spectrum for Windows (registered trademark)Ver. 1.4, and the absorbance of the peak at 1809 cm⁻¹ was measured.Also, the film thickness was measured with a micrometer.

Measurement Method for the Number of Carboxylic Acid Fluoride Groups

In the same way as with the above measurement method for the number ofcarbonate groups, by infrared spectrum analysis of the resulting film,the peak originating in the carbonyl group of the carboxylic acidfluoride group (—C(═O)F) appeared at an absorption wavelength of 1880cm⁻¹ (v_(c-o)), and the absorbance thereof at the v_(c-o) peak wasmeasured. The number of carboxylic acid fluoride groups was measured inthe same way as in the above measurement method for the number ofcarbonate groups using the above formula (5), except that from modelcompounds. the mole absorbance coefficient (l·cm⁻¹·mol⁻¹) of the v_(c-o)peak of the carboxylic acid fluoride group was set to ∈=600.

Measurement Method for the Number of Other Carbonyl Groups

In the same way as with the above measurement method for the number ofcarbonate groups, infrared spectrum analysis of the resulting film canbe used to measure the number of other carbonyl groups that canbasically react with amide groups, amino groups, and other functionalgroups in a polyamide resin such as carboxylic acid groups, estergroups, acid anhydride groups, and the like. Here, except for settingthe mole absorbance coefficient (l·cm⁻¹·mol⁻¹) of the v_(c-o) peakoriginating in these carbonyl groups to ∈=530, the number of othercarbonyl groups was measured in the same way as in the above measurementmethod for the number of carbonate groups using the above formula (5).

Measurement Method for the Composition of the Fluorine-ContainingEthylenic Polymer

Measurements were made by ¹⁹F-NMR analysis.

It is preferable that the fluorine-containing ethylenic polymer in thepresent invention be used singly, so as not to detract from theadhesiveness, heat resistance, resistance to chemicals, and otherproperties that it has itself, but insofar as its performance is notdegraded, it may according to purpose and application be blended withvarious well known fillers such as inorganic powder, glass fiber, carbonfiber, metal compounds, carbon, or the like. Moreover, besides fillers,one may also mix in other additives as desired, such as pigments andultraviolet ray absorbents. Besides additives, one may also blend inother fluororesins or thermoplastic resins, thermoplastic and otherresins, synthetic rubber, and the like, thereby making it possible toimprove the mechanical properties, improve the weather resistance, adddecorative designs, prevent static electricity, improve the moldability,and the like.

As a result of combining the above polyimide film with the abovefluororesins, the coverlay of the present invention has excellentthermal shrinkage and plenty of adhesive strength. The coverlay of thepresent invention at the least is formed by laminating the fluororesinto the polyimide film in an adhesive state. Various manufacturingmethods can be applied to the manufacture of the coverlay, including amanufacturing method of forming, one after another or by extrusiontogether, the constituent layers that include the polyimide film and thefluororesin, a manufacturing method of thermocompression bonding of amolded body; and a manufacturing method of taking either the polyimidefilm or the fluororesin as a molded body that is coated with a precursoror molten version of the other resin, and is allowed to flow along tomake a resin composition; and a good adhesive state is formed betweenthe constituent layers, which include the polyimide film and thefluororesin. In this manufacturing, one may use any well known moldingmachine that is normally used for thermoplastic resin, such as aninjection molding machine, a compression molding, a flow moldingmachine, or an extrusion molding machine.

The forming conditions vary with the carbonyl group, especially the typeof carbonate group, and with the type of fluorine-containing ethylenicpolymer, but with extrusion or flow molding it is appropriate to heatthe cylinder to a temperature of 200° C. or more. It is preferable thatthe heating temperature be set to no greater than a temperature thatwill suppress foaming or other bad effects caused by the thermalbreakdown of the fluorine-containing ethylenic polymer itself;specifically, 400° C. or less is preferable, and 350° C. or less is morepreferable.

There are no particular restrictions on the manufacturing method throughthermocompression bonding, and examples include the methods of vacuumpressing, lamination (the hot laminate method, and the like), andcoating. Also, the fluorine-containing resin layer may includelamination and coating on either one side or both sides of the polyimidefilm.

With a vacuum press, a coverlay is obtained by, for example,thermocompressing together the polyimide resin and the fluororesin atthe prescribed temperature and pressure using a well known vacuum pressmachine. To make the processing simple, it is preferable when doing thisthat the press temperature be in the range of 100 to 250° C. Annealingmay be done following the pressing, and it is preferable that theannealing temperature be in the range of 100 to 250° C.

With the hot laminate method, on which there are no particularrestrictions, a coverlay is obtained by using two heatable rollers whosedistance between them can be adjusted as desired, layering film of twoor more types between the rollers, and pressing them together whileapplying heat and pressure. If necessary, heat treatment can be donecontinuously immediately after the laminating is done. In the heattreatment, it is preferable that the temperature be no less than theglass transition point (Tg) of the fluororesin and no greater than itsmelting point +50° C., because this allows the pressing-together forceto be increased. A temperature below Tg is undesirable because then thedesired bonding force cannot be obtained, and a temperature above themelting point+50° C. is undesirable because then the fluororesin beginsto break down, lowering the bonding force. There are no particularrestrictions on the heating time; it can be set suitably as necessary.There are no particular restrictions on the equipment, as long as itdoes not hamper the effects of the present invention.

The adhesive strength between the polyimide layer and the fluororesinlayer of the coverlay before the copper-clad laminate is attached ispreferably more than 3.0 N/cm, more preferably at least 5.0 N/cm, andeven more preferably at least 8.0 N/cm; this is for sake of improvingthe precision in the positioning of the copper-clad laminate andcoverlay and raising the operational efficiency. There are no particularrestrictions on the upper limit for the adhesive strength.

Although there are no particular restrictions on the thickness of thepolyimide layer in the coverlay of the present invention, because thisthickness affects the bonding force of the coverlay's polyimide layerand fluororesin layer, it is preferably about 0.01 to 2.0 times thethickness of the fluororesin layer, more preferably about 0.05 to 1.0times, and even more preferably about 0.1 to 0.9 times. A thickness ofthe polyimide layer that exceeds a factor of 2.0 is undesirable because,although it improves the rigidity and dimensional stability for thesubstrate, it increases the dielectric constant. Moreover, if it is lessthan a factor of 0.01, the rigidity of the polyimide layer willdecrease, the coefficient of linear expansion will tend to increase, andthe rigidity and dimensional stability as a substrate will decrease.

The thermal shrinkage of the coverlay of the present invention asmeasured at 260° C. at 30 minutes is normally less than ±0.1%,preferably less than ±0.08%, and more preferably less than ±0.06%.

A high-frequency circuit substrate can be manufactured by affixing thecoverlay of the present invention to a copper-clad laminate. There areno particular restrictions on how to make the copper-clad laminate; itmay be manufactured by any well known method. Also, the copper-cladlaminate may have either a one-sided structure or a two-sided structure.

As manufacturing methods for the copper-clad laminate to be affixed tothe coverlay of the present invention, examples include a three-layerCCL in which a base-material film and copper foil are laminated togetherwith an intervening adhesive, a method in which a copper layer is formedby making use of vapor deposition onto the base-material film along withsputtering and electroplating, a so-called cast-type two-layer CCL (COC)in which a polyimide layer is cast and formed on copper foil, and thecopper layer is formed using electroless plating on the base-materialfilm.

As the base-material film, examples include polyimide film and LCP filmfor use with high-frequency circuits. In addition, as the adhesivelayer, examples include epoxy, acrylic, or polyimide adhesives, as wellas fluororesins, and the like. Commercially available products may beused for the adhesive. As commercially available products, there are noparticular restrictions, and examples include the LF Series (of acrylicadhesives) of Pyralux (made by Dupont Co., Ltd.), and the like.Preferable modes among these are copper-clad laminates that make use ofLCP film, and copper-clad laminates in which the laminate is made withthe fluororesin between the base-material film and the copper foil.

As the polyimide film to be used for the copper-clad laminate, onessimilar to the above polyimide film for the coverlay can be cited, andtheir composition may be either the same as or different from thepolyimide film for the coverlay.

There are no particular restrictions on the fluororesin to be used inthe copper-clad laminate; any well known fluororesin may be used,including commercially available products. As such commerciallyavailable products, examples include Toyoflon F, FE, FL, FR, and FV(brand names; made by Toray Advanced Film Co., Ltd.), and the like.

There are no particular restrictions on the thickness of the polyimidelayer in the copper-clad laminate (the layer including the adhesive, ifa polyimide adhesive is used), but a thickness 0.01 to 2.0 times thethickness of the fluororesin layer is preferable, about 0.05 to 1.0times is more preferable, and about 0.1 to 0.9 times is even morepreferable. A thickness of the polyimide layer that exceeds 2.0 timesthe thickness of the fluororesin layer is undesirable because althoughthe rigidity and dimensional stability as a copper-clad laminate willimprove, the dielectric constant will increase. Moreover, if it is lessthan a factor of 0.01, the rigidity of the polyimide layer willdecrease, the coefficient of linear expansion will tend to increase, andthe rigidity and dimensional stability as a copper-clad laminate willdecrease.

A wiring-processed copper-clad laminate is obtained by etching thecopper-clad laminate. There are no particular restrictions on how theetching is to be done, and any well known method may be used.

In manufacturing a high-frequency circuit substrate, laminating is donein such a way that the fluororesin side of the coverlay comes intocontact with the circuit of the copper-clad laminate, and the coverlayand the copper-clad laminate are held temporarily in place.

There are no particular restrictions on the step by which thecopper-clad laminate and the coverlay are held temporarily in place, andany well known method may be used; examples include a method in whichthe copper-clad laminate and the coverlay are aligned in position, kisslamination is done, then as necessary, quick-pressing is done to producelamination at about 150 to 200° C., a method in which the copper-cladlaminate and the coverlay are aligned in position, and multi-stagepressing is done, and the like. There are no particular restrictions onthe maximum temperature in the step for temporarily holding in place, aslong as it is lower than the melting point of the fluororesin that is tobe used in the coverlay, but to obtain sufficient adhesive strength bysubsequent annealing, it is appropriate that it be in the range of 100to 250° C. There are no particular restrictions on the treatment timefor the step of temporarily holding in place.

There are no particular restrictions on the manufacture of ahigh-frequency circuit substrate, but following the step of temporarilyholding in place, it is preferable to execute a step in which thecopper-clad laminate to which the coverlay is attached is annealed.

There are no particular restrictions on the maximum heating temperaturein the annealing step, but for good adhesive strength of the resultinghigh-frequency circuit substrate, it is preferable to set it to atemperature within the range from 150° C. to 350° C. at a temperaturethat is higher than the maximum temperature in the step for temporarilyholding in place, and it is preferable that the temperature differencebe at least 20° C. Moreover, in the annealing step, it is preferablethat it be done by free tension between 150° C. and 350° C. Free tensionhas the advantage of simplifying the treatment steps, and with regard tothe annealing temperature, from 200° C. to 280° C. is more preferable,and from 205° C. to 275° C. is even more preferable. There are noparticular restrictions on the annealing time.

The annealing increases the bonding force based on the adhesive force ofthe fluororesin layer, and results in a high-frequency circuit substratethat has practical adhesive strength (peel strength). To ensureperformance as a coverlay, the adhesive strength after annealing of theresulting high-frequency circuit substrate preferably is a value thatexceeds 8 N/cm, and more preferably is at least 10 N/cm. It is even morepreferable that it be at least 14 N/cm. The adhesive strength in thepresent invention is the value that was measured by the method describedbelow in the working examples.

A high-frequency circuit substrate can be manufactured by laminatingtogether the coverlay of the present invention and a copper-cladlaminate that has been wiring processed. Making the thickness of thepolyimide film and the thickness of the fluororesin so that they havethe above-specified ratio not only further improves the electrical andmechanical properties but also ensures excellent dimensional stability,further repressing the occurrence of curling, twisting, warping, and thelike even if one carries out etching for circuit formation of the copperlayer, and various heating steps in the steps following circuitformation.

WORKING EXAMPLES

Next, the present invention is described in greater specificity, citingworking examples, but the present invention is not restricted by theworking examples thereof, and many modifications can be made by a personwho has the usual knowledge in this field, within the technical conceptof the present invention.

The following is a description of the methods for measuring the variousproperties in the present invention.

(1) Peel Strength

A sample was cut into a strip 10 mm wide, and the peel strength (unit:N/cm) was measured in a 90° C. [sic; 90 degrees] pulling test (pullingspeed: 50 mm/min, measurement length: 20 mm, measurement range: 5.0 to20.0 mm) using the Autograph AG-IS universal tensile testing device madeby Shimadzu Ltd.

(2) Thermal Shrinkage

A sample of size 190 mm×200 mm was cut, and its dimensions before heattreatment were measured with the CNC image processing device systemNEXIVVM-250 made by Nikon Co., Ltd. Then the sample was put into an ovenset to 260° C. and heat treated for 30 minutes. The sample after heattreatment was moisture-adjusted for 12 hours or more at a constanttemperature and high humidity. The sample after moisture adjustment wasmeasured for its dimensions in the same way as before the heattreatment, and the rate of change in the dimensions before and after theheat treatment was shown as a percentage.

(3) Transmission Loss

The high-frequency transmission characteristics from 1 to 40 GHz weremeasured using a prober device for substrate measurement made by CascadeMicrotech.

Synthesis Example 1

Pyromellitic acid dianhydride (molecular weight218.12)/3,3′,4,4′-biphenyl tetracarboxylic acid dianhydride (molecularweight 294.22)/4,4′-diaminodiphenyl ether (molecular weight200.24)/paraphenylene diamine (molecular weight 108.14) was prepared atmole ratio of 95/5/85/15, and was made into a 20 wt % solution in DMAc(N,N-dimethyl acetoamide) and polymerized, and a 3500-poise polyamicacid solution was obtained.

Synthesis Example 2

Pyromellitic acid dianhydride (molecular weight218.12)/4,4′-diaminodiphenyl ether (molecular weight 200.24) wasprepared at a mole ratio of 100/100, and was made into a 20 wt %solution in DMAc (N,N-dimethyl acetoamide) and polymerized, and a3500-poise polyamic acid solution was obtained.

Synthesis Example 3

A 380-liter quantity of distilled water was put into an autoclave, andafter carrying out nitrogen replacement sufficiently, it was chargedwith 75 kg 1-fluoro-1,1-dichloroethane, 155 kg hexafluoropropylene, and0.5 kg perfluoro(1,1,5-trihydro-1-pentene), and the interior of thesystem was kept at 35° C. at a stirring speed of 200 rpm. Next,tetrafluoroethylene was pressurized to 0.7 MPa, and subsequentlyethylene was pressured to 1.0 MPa, then 2.4 kg di-n-propylperoxydicarbonate was inserted, and polymerization was initiated.Because the pressure within the system decreases as the polymerizationproceeds, the pressure within the system was kept to 1.0 Mpa bycontinuously supplying a mixed gas made oftetrafluoroethylene/ethylene/hexafluoropropylene=40.5/44.5/15.0 mol %.Then, for the perfluoro(1,1,5-trihydro-1-pentene) as well, a totalquantity of 1.5 kg was charged, and stirring was continued for 20 hours.Then, after the pressure was released and the system returned toatmospheric pressure, the reaction products were washed with water anddried, producing 200 kg of powder (fluorine-containing ethylenic polymerF-A). The analysis results thereof are presented in Table 1.

Synthesis Example 4

In the same way as in Synthesis Example 3, fluorine-containing ethylenicpolymer F—B was obtained in the blends shown in Table 1. The analysisresults thereof are presented in Table 1.

Synthesis Example 5

A 9.5 kg quantity of powder of the fluorine-containing ethylenic polymerF—B obtained in Synthesis Example 4, 700 g of 28% aqueous ammonia, and10 liter of distilled water were charged into an autoclave, the systemwas heated while stirring, and kept at 80° C., and the stirring wascontinued for 7 hours. Then the content was water-washed and dried,yielding 9.2 kg of powder (fluorine-containing ethylenic polymer F—C).By carrying out such treatment, the active functional groups containedin the resin (carbonate groups and carboxylic acid fluoride groups) weretransformed into amide groups that are stable both chemically andthermally. Also, it was confirmed by infrared spectrum analysis thatthis transformation proceeded quantitatively. The analysis results ofthe resin after treatment are presented in Table 2. Also, no carbonylgroups except carbonate groups and carboxylic acid fluoride groups werefound in the fluorine-containing ethylenic polymer (F-A) shown inSynthesis Example 3. In Table 1, TFE represents tetrafluoroethylene, Etrepresents ethylene, HFP represents hexafluoropropylene, and HF-Parepresent perfluoro(1,1,5-trihydro-1-pentene).

TABLE 1 Quantity (number) per 10⁶ carbon Fluoro- atoms in main resinchain (fluorine- Monomer composition carboxylic MFR containing (mol %)acid Melting (g/10 min) ethylenic HF- carbonate fluoride point (measuredpolymer TFE Et HFP Pa groups groups (° C.) temperature) SE 3 F-A 40.844.8 13.9 0.5 300 3 162.5 2.6 (230° C.) SE 4 F-B 46.2 43.8 9.5 0.5 255 5194.3 8.9 (230° C.) SE 5 F-C 46.1 43.8 9.5 0.5 not detected not detected193.5 9.8 (230° C.) SE = Synthesis example

Working Example 1 (1) Preparation of the Polyimide Film

Acetic anhydride (molecular weight 102.09) and β-picoline were mixed inthe polyamic acid solution obtained in Synthesis Example 1 at respectiveratios of 17 wt % and 17 wt % with respect to the polyamic acidsolution, and stirred. The resulting mixture was cast by a T-shaped slitdie onto a rotating stainless steel drum at 75° C., and after themixture was allowed to flow and extend for 30 seconds, the resulting gelfilm was stretched 1.2-fold in the running direction while being heatedfor 5 minutes at 100° C. Next, both ends in the width direction weregripped, and the film was stretched 1.3-fold in the width directionwhile heating for 2 minutes at 270° C., after which it was heated for 5minutes at 380° C., producing a polyimide film with a thickness of 12.5μm.

(2) Preparation of the Fluororesin Film

The fluorine-containing ethylenic polymer polymerized in SynthesisExample 3 was pelletized using a 65-mm-diameter short-axis extrudingmachine with a temperature setting of 230° C. to 280° C., then it wasmade into a film using a 50-mm-diameter short-axis extruding machineequipped with a T-die and having a temperature setting of 230° C. to280° C., thereby producing a fluororesin film with a thickness of 25 μm.

(3) Preparation of the Coverlay

Using the polyimide film obtained in (1) above and the fluororesin filmobtained in (2) above, a coverlay was made by the vacuum press method.Specifically, the polyimide film and the fluororesin film were laid atopeach other and then pressed for 90 seconds at 120° C. and 30 kN with avacuum press machine, after which an electric furnace set to 180° C. wasused to heat the material for 20 minutes with free tension, therebyproducing a coverlay film. The above-described properties were thenmeasured for the resulting coverlay, and the results are shown in Table2.

Working Example 2

A coverlay was made in the same way as in Working Example 1, with theexception that instead of the polyimide film obtained in (1) of WorkingExample 1, a polyimide film made in the same way as in (1) of WorkingExample 1 using the polyamic acid solution obtained in Synthesis Example2 was used, and instead of the fluorine-containing ethylenic polymer(F-A) obtained in Synthesis Example 3, the fluorine-containing ethylenicpolymer (F—B) obtained in Synthesis Example 4 was used. Each of theabove-described properties was measured, and the results are presentedin Table 2.

Comparison Example 1

A coverlay was made using the same method as the preparation method of(3) in Working Example 1 and using the polyimide film obtained in (1) ofWorking Example 1 and the fluorine-containing ethylenic polymer (F—C)obtained in Synthesis Example 5.

TABLE 2 Working Working Comparison Item Unit Example 1 Example 2 Example1 Peel strength [N/cm] 3.1 3.6 0.1 or less Thermal [%] 0.05 0.09 0.18shrinkage(In the table, each coverlay peel strength denotes the adhesive strengthbetween the polyimide film layer and the fluororesin layer.)

In Comparison Example 1, it took time to align the positions of thecopper-clad laminate and the coverlay, which is not suitable forindustrial implementation. Moreover, it was learned that, as shown inTable 2, in Comparison Example 1, sufficient bonding force is notobtained, which makes it unsuitable industrially. On the other hand,with the coverlay of the present invention, the workability wasexcellent, and the thermal shrinkage was less than ±0.1%.

Preparation 1

A two-sided CCL was fabricated by using an epoxy adhesive to attach an18 μm copper foil to the polyimide film obtained in (1) of WorkingExample 1.

Working Example 3

A coverlay was fabricated by the method shown in (3) of Working Example1, using polyimide film (thickness: 12.5 μm) obtained in the same way asin (1) of Working Example 1 and the fluorine-containing ethylenicpolymer (F—B) obtained in Synthesis Example 4.

Comparison Example 2

A coverlay was fabricated by using a bar coater to coat one side of apolyimide film (thickness: 12.5 μm) obtained in the same way as in (1)of Working Example 1 with epoxy adhesive to a thickness of 25 μm,heat-drying it for 5 minutes at 150 degrees [sic; degrees Celsius],performing B staging, and then bonding the separate film to the resincomposition surface with a laminator.

Test Example Transmission Properties

Using the CCL fabricated in Working Example 1, etching was done toproduce the desired wiring, and using the etched CCL and the coverlay ofWorking Example 3 or the coverlay of Comparison Example 2, a circuit wasfabricated, and the transmission properties thereof were measured. Themeasurement results are shown in FIG. 1.

As described above, when the coverlay of the present invention was used,a high-frequency circuit substrate was obtained that exhibits superiorworkability when manufacturing high-frequency circuit substrates, andthat has excellent mechanical properties and heat resistance. Moreover,as shown in FIG. 1, the coverlay of the present invention also hasbetter properties than a conventional coverlay, including transmissionproperties.

INDUSTRIAL POTENTIAL

The coverlay of the present invention requires no high-temperaturepressing beforehand, and by low-temperature pressing and subsequentheating with free tension, one can obtain a FPC [flexible printedcircuit] that has excellent high-frequency properties, dimensionalstability, and wiring precision. Also, because it has a low dielectricconstant, the high-frequency circuit substrate of the present inventioncan keep transmission loss in check.

What is claimed is:
 1. A coverlay for a high-frequency circuitsubstrate, the coverlay comprising a polyimide film and a fluororesinbonded together, and an adhesive strength between the polyimide filmlayer and the fluororesin layer being greater than 3.0 N/cm.
 2. Thecoverlay as described in claim 1, wherein a thermal shrinkage thereof at260° C. for 30 minutes is less than ±0.1%.
 3. The coverlay as describedin claim 1 or 2, wherein the fluororesin has a melting point of 200° C.or less.
 4. The coverlay as described in any one of claims 1 through 3,wherein the fluororesin is a fluorine-containing ethylenic polymer, andthe fluorine-containing ethylenic polymer contains a carbonyl group. 5.The coverlay as described in claim 4, wherein a quantity of carbonylgroups contained in the fluorine-containing ethylenic polymer totals 3to 1000 groups per 1×10⁶ main-chain carbon atoms.
 6. The coverlay asdescribed in any one of claims 1 through 3, wherein the fluororesin ismade up of fluorine-containing ethylenic polymer that has at least onetype selected from a group made up of carbonate groups, carboxylic acidhalide groups, and carboxylic acid groups totaling 3 to 1000 groups per1×10⁶ main-chain carbon atoms.
 7. The coverlay as described in any oneof claims 1 through 3, wherein the fluororesin is one or more types offluorine-containing ethylenic monomer selected from a group made up oftetrafluoroethylene, vinylidene fluoride, chlorotrifluoroethylene, vinylfluoride, hexafluoropropylene, hexafluoroisobutene, monomers representedby the following formula (X):CH₂═CR¹(CF₂)_(n)R²  (X) (wherein R¹ represents H or F, R² represents H,F, or Cl, and n is a positive integer in a range 1 to 10), andperfluoro(alkyl vinyl ethers) having 2 to 10 carbon atoms, or afluorine-containing ethylenic polymer made by polymerizing thefluorine-containing ethylenic monomer and an ethylenic monomer having 5or fewer carbon atoms.
 8. The coverlay as described in any one of claims1 through 3, wherein the fluororesin is a copolymer made by polymerizingat least the following (a), (b), and (c); (a) 20 to 90 mol % oftetrafluoroethylene, (b) 10 to 80 mol % of ethylene, and (c) 1 to 70 mol% of a compound represented by the formula:CF₂═CFR³  (Y) (wherein R³ represents CF₃ or OR⁴, and R⁴ represents aperfluoroalkyl group having 1 to 5 carbon atoms).
 9. The coverlay asdescribed in any one of claims 1 through 8, wherein the polyimide filmis made up mainly of one or more aromatic diamine components selectedfrom a group made up of paraphenylene diamine, 3,4′-diaminodiphenylether, and 4,4′-diaminodiphenyl ether, and one or more acid anhydridecomponents selected from a group made up of pyromellitic aciddianhydride and 3,3′,4,4′-biphenyl tetracarboxylic acid dianhydride.